This invention relates to exothermic resistor elements, and to a thermal process air flow meter of the type used to measure forward and backward air flows in internal combustion engines, and the like.
Thermal process flow meters are frequently used for electronically controlled fuel injection systems for internal combustion engines, because of their advantages such as compact size, quick response, etc. Such air flow meters are disclosed, for example, in Japanese Patent Applications Laid-Open No. 265118/1988 and No. 185416/1989, as well as in U.S. Pat. No. 5,086,650.
In conventional exothermic resistor elements, electrode terminals are provided on both the right and left ends, with the number of terminals being four in some cases and three in others. When three electrode terminals are provided, they are arranged on opposite walls of a suction passageway, which complicates the connection of the exothermic resistor element to the electric circuit. Those elements also occupy excessive space for mounting.
In addition, in conventional thermal process air flow meters, if both the first and second exothermic resistors are arranged parallel to the direction of air flow, on the same surface of a substrate, the difference in their respective heat transmission is small. When they are inclined relative to each other, the difference in the heat transmission is less affected by turbulence, but it is impossible to generate a significant difference between the amount of radiated heat. As a result, the heat response is slowed when the air flow reverses, which makes it difficult to detect the direction of air flow accurately. This problem has remained unsolved heretofore.
U.S. Pat. No. 5,086,650 discloses an air flow detector in which the substrate is inclined relative to the air flow direction, and bi-directional measurement is achieved by placing a first exothermic air flow detector to measure air flow in a forward direction on one side of the substrate, and a second (identical) exothermic air flow detector to measure reverse air flow on the other side. Each of the respective detectors has a heater element situated adjacent to heat detectors, and air flow is measured by pulsing the heater and measuring the phase shift in the output signal of a downstream detector. This arrangement achieves bidirectional air flow detection but requires a complex configuration of heater and sensors as well as complex circuitry.